Gasoline powered spark ignition internal combustion engines and particularly light duty and small engines are used on a large variety of products including handheld, lawn and garden, marine, snowmobile and other home and commercial products. These engines are typically two-cycle or four-cycle engines with one or more cylinders and have a spark plug for each cylinder which in use initiates combustion of a fuel-and-air mixture in the cylinder. The spark plug is typically threaded or otherwise secured in a bore in a metal cylinder head or cylinder of the engine which provides a ground for a metal shell or body of the spark plug which has an electrical ground electrode adjacent one end and for installing or removing the spark plug a non-circular and typically hexagonal nut portion adjacent its other end. An electrically conductive center electrode typically with a copper core extends through the metal body with one end spaced by a gap from the ground electrode and is received in a typically ceramic insulator which projects from the other end of the body and carries an electrically conductive terminal connected to the center electrode.
In use, though an insulated wire with an end clip removably connected to the terminal, a high potential voltage current is supplied to the center electrode to produce an arc or spark in the gap. Typically, an electrically insulating boot is generally coaxially received over the terminal and an exposed portion of the insulator of the spark plug and terminates short of or adjacent the upper end of the spark plug shell or body. Typically, the boot has an integral arm portion through which the insulated electric wire extends and this arm portion typically is inclined at an acute included angle usually of about 90° or 45° to the longitudinal axis of the main body of the boot and the spark plug. In many small engine applications, in use the high potential voltage is supplied to this wire by a so-called switch or module controlling the ignition timing which is typically part of an electromagneto capacitive discharge ignition system.
When in use in an operating engine, the arcing or spark produced by the spark plug creates electromagnetic interference (EMI) which may adversely affect the circuitry of the module controlling ignition timing and/or other engine operations which adversely affects engine performance or it may adversely affect other electronic circuitry of the product on which the engine is used or in some instances other devices or products in the vicinity in which the engine is operating.